This invention relates to catalyst deactivation and removal of catalyst residues from polymers.
Various processes for polymerizing olefins are described in the literature. A typical approach is to carry out the polymerizations in the presence of a transition metal compound catalyst and an organometallic compound cocatalyst. One of the problems encountered with polymers prepared by such processes concerns the presence in the polymer of catalyst residues. The presence of these catalyst residues in the polymer adversely affects the heat stability, color, electrical properties and, in the case of clear resins, transparency. The halide components of catalyst residues can cause corrosion of processing and molding equipment.
For polyolefins which require high processing temperatures, catalyst removal is even more critical since higher temperatures may magnify adverse effects of the catalyst residues.
The process by which catalyst residue impurities are removed, or extracted, from a polyolefin is referred to as "deashing". Various methods for extracting catalyst residues from polyolefins have been developed. These commonly involve treating the polymer with a variety of agents and adjuvants. Treating agents such as dicarbonyl compounds which contain the group ##STR1## used in conjunction with alkylene oxide adjuvants have been shown to be useful for extracting catalyst residues from polyolefins such as polypropylene. (See U.S. Pat. No. 3,219,647). The dicarbonyl compounds also serve to deactivate the catalyst and cocatalyst thereby terminating the polymerization reaction. Additionally, the dicarbonyl compounds enhance the solubility of the catalyst residues for the subsequent extraction process. A subsequent rinse or wash of the polymer with an alcohol, a paraffinic hydrocarbon or monomer facilitates extraction.
The chelating reaction of dicarbonyl compounds with metal halide catalysts converts the metal to an extractable form and is accompanied by the evolution of hydrogen halide. Metal constituents of the organometallic cocatalyst also react with dicarbonyl compounds in this way. Depending on the nature of the cocatalyst, this reaction may also be accompanied by the evolution of hydrogen halide.
The alkylene oxide treating agent is employed as an adjuvant to remove the hydrogen halide which would otherwise inhibit the chelating reaction, thereby slowing down the rate of metal extraction.
One problem area encountered when using conventional methods for deashing catalysts in polymer production is the necessity of multiple separations to recover unreacted monomer from the reaction solution after the polymer product is separated. This is especially a problem when polymers of branched higher alpha olefins are being made because of the relative boiling points of the unreacted monomer and the alkylene oxides. Use of low boiling alkene oxides with dicarbonyl compounds for deashing polymers formed in polymerization of 1-olefin monomers with transition metal/organometal catalysts requires use of an additional separation column to separate the light (low boiling) alkylene oxides from the branched olefin monomer.
Another problem which has been observed when using conventional methods to remove catalyst residues from polyolefins of branched 1-olefin monomers, particularly those monomers with at least one substituent in the three position of the olefin, is the relatively high level of catalyst residues in the polymer. This problem may be due to low productivity in the polymerization process. Polymerizations of branched 1-olefins are typically low yield reactions due to the sterically hindered structure of the monomer. As a result, catalyst residues will exist at high levels in the polymer. Thus, effective deashing methods are important.